The power delivered by a power amplifier to a load resistance depends on the voltage standing wave ratio (VSWR), that is, the change in the magnitude and phase of the reflection coefficient referenced to the designed load resistance, also referred to as the nominal load resistance or the nominal load impedance. System requirements that demand the leveling of this output power as the load impedance changes present difficult circuit design problems which are particularly troublesome in a cost sensitive environment. Measuring the VSWR directly at the output of the power amplifier, using well-known radio frequency (RF) VSWR measurement methods, adds cost in terms of a dual directional coupler, detection diodes or dual logarithmic amplifiers, and other overhead such as additional analog to digital conversion(s).
Many applications require an output signal with a consistent power level. One such application is a cellular network in which a cellular telephone includes a power amplifier coupled to an antenna. The antenna in this case is the load. FIG. 1 illustrates an exemplary configuration in which a power amplifier 10 is coupled to an antenna 30. An RF input signal is input to the power amplifier 10. A supply voltage is provided from the power supply 20 to power the power amplifier 10. The power amplifier amplifies the input RF signal and outputs an amplified RF output signal which is transmitted by the antenna 30. Under ideal conditions, the actual output load impedance at the output of the power amplifier is the same as the nominal load impedance Z0, as illustrated in the FIG. 2A. In this case, the RF output signal is transmitted from the antenna 30 at an output power that is within expected parameters.
However, as is well known in the art, the RF output signal transmitted via the antenna can vary in power from its nominal operating parameters. In such cases, the actual output load impedance is some load impedance Z1 different than the expected nominal load impedance Z0. This is referred to as a mismatched load impedance. In many cases, such a mismatch can be quite substantial and will negatively impact the performance of the system. FIG. 2B illustrates a mismatched load impedance condition. Whenever the actual output load impedance Z1 is different than the nominal load impedance Z0, a portion of the output power is reflected back toward the power amplifier 10. The remaining portion of the output power is transmitted through the antenna 30, whereby the output power of the transmitted signal is reduced. Many systems require a consistent transmitted signal, or a signal power within a particular range. Additionally, the reflected power is potentially harmful to the power amplifier, leading to possible damage or destruction.
To compensate for mismatch load conditions, it is necessary to quantify the level of load mismatch and then to adjust the power input to the power amplifier accordingly. A conventional method of quantifying the load mismatch is to add a detection circuit at the output of the power amplifier. FIG. 3 illustrates a conventional circuit configuration for measuring a mismatched load impedance of a power amplifier. A detection circuit is added at the output of the power amplifier 10. The detection circuit includes a coupler 40, a first diode 50, and a second diode 60. The detection circuit functions to measure the amount of power Pf in the forward direction, that is, from the power amplifier 10 to the antenna 30, and to measure the amount of power Pr in the reverse direction, that is, from the antenna 30 to the power amplifier 10. The reverse power Pr is the reflected power resulting from the mismatched load impedance. The measured forward power Pf and the reverse power Pr are used to calculate the actual output load impedance, which in turn is used to adjust the supply voltage provided by the power supply 20 to the power amplifier 10. Ideally, the reverse power Pr is zero, and the forward power Pf is the nominal output power.
There are many problems associated with using a detection circuit, such as that of FIG. 3, at the output of the power amplifier. There are many additional parts which adds complexity, size, and cost. Each of the additional parts exhibits performance characteristics that may or may not be known, there are additional manufacturing considerations associated with the additional parts, and some of the additional parts, such as the diodes, are temperature dependent. Further, the detection circuit results in some power loss, so that even under ideal conditions the output power is reduced leading to an overall drop in efficiency. Also, use of a detection circuit requires additional logic circuitry necessary to perform the additional calculations. These are just a sampling of the problems associated with conventional detection circuits.